U.S. Pat. No. 6,869,474 to Perez-Pena et al., incorporated herein by reference, discusses extremely fast setting of cementitious compositions for producing cement-based products such as cement boards achieved by adding an alkanolamine to a hydraulic cement such as Portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90° F. (32° C.). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. The extremely rapid set permits rapid production of cementitious products. Triethanolamine additions have been found to be a very powerful accelerator capable of producing formulations with relatively short final setting times with increased levels of fly ash and gypsum and without the need of calcium aluminate cements. However, formulations with triethanolamine also had relatively lower early-age compressive strength compared to cement board formulations containing the calcium aluminate cements.
U.S. Pat. No. 4,488,909 to Galer et al., incorporated herein by reference, discusses cementitious compositions capable of rapid setting. The compositions permit high speed production of carbon dioxide resistant cement boards by forming essentially all of the potential ettringite within about 20 minutes after the composition is mixed with water. The essential components of the cementitious composition are portland cement, high alumina cement, calcium sulfate and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth and pumicite may be added up to about 25%. The cement composition includes about 14 to 21 wt % high alumina cement, which in combination with the other components makes possible the early formation of ettringite and other calcium aluminate hydrates responsible for early setting of the cementitious mixture. In their invention, Galer et al. provided aluminates using high alumina cement (HAC) and sulfate ions using gypsum to form ettringite and achieve rapid setting of their cementitious mixture.
Ettringite is a calcium aluminum sulfate compound having the formula Ca6Al2(SO4)3.32 H2O or alternatively 3 CaO.Al2O3.3 CaSO4.32 H2O. Ettringite forms as long needle-like crystals and provides rapid early strength to cement boards, so that they can be handled soon after being poured into a mold or over a continuous casting and forming belt.
In general, Galer et al's rapid setting formulation suffers from several limitations. These limitations, as highlighted below, are even more of a concern for the production of cementitious products such as cement boards.
The final setting times of the cementitious mixtures are typically greater than 9 minutes. The final setting time is defined further in the examples below, but more generally, the cementitious mixtures have set to the extent that the cement-based products made thereof can be handled and stacked, although chemical reactions may continue for extended periods.
The amount of high alumina cement (also known as calcium aluminate cement) in the reactive powder blend is very high. Typically, the high alumina cement is greater than 14 wt % of the reactive powder blend.
The amount of pozzolanic materials is limited to 25 wt % of the reactive powder blend.
Lime is required as an additional ingredient to obtain rapid set. Presence of excess lime in cement boards is detrimental to their long-term durability performance. Cement boards often are reinforced with polymer coated glass fiber mesh that degrades, losing strength and ductility in a high alkaline environment. Presence of excess lime increases the alkalinity of the cementitious matrix and thereby negatively impacts the long-term durability performance of the polymer coated glass fiber mesh and the resulting cement boards. In addition, presence of excess lime also makes the concrete susceptible to sulfate attack thus influencing its durability.